As a person's age increases, typically the portion of light that is scattered when it passes through the human eye lens increases, as well. This may cause, inter alia, cloudiness in the eye lens that is also known as a “gray star” or “cataract.” Since such cloudiness of the ocular lens very sharply diminishes the person's visual acuity, with older patients frequently the cloudy human lens is replaced by a clear, synthetic intraocular lens.
In every imaging process, cloudiness in the eye lens (cataract), in the vitreous body, and/or in the cornea has a negative impact on image quality in that it scatters both the incident illumination light and the light reflected in the eye. Inclusions or foreign bodies in the eye have the same effect. Even the ophthalmological device itself can cause scattered light, for instance due to soiled optical surfaces. Scattering of light from a cataract is significantly more pronounced for blue light than for light at other wavelengths.
Scattered light that occurs thus also influences the results of important diagnostic procedures. For instance, when measuring the optical density of the macular pigment xanthophyll on the ocular fundus, in accordance with the description of WO 2009/046912 A1, the measured results are distorted due to scattered light that occurs.
WO 2009/046912 A1 suggests a method that determines the macular pigment optical density (MPOD) in the region of the fovea. For this, the brightness curve on the fundus (IB) is determined in the area with no pigment and compared to the actual measured brightness (IM). The optical density of the pigment (OD) is then calculated as follows:OD=0.5*log (IB/IM)
However, when heavy scattered light IS occurs, the measurement results are distorted, because the values (IB+IS) and (IM+IS) are measured instead of the true values IB and IM. The distortion of the true values IA and IM increases as the intensity of the scattered light IS increases.
In the case of a very severe cataract, in very dark regions of the fundus, for instance the fovea, the scattered light may attain nearly the same intensities as the actual measurement signal reflected by the fundus. The method described thus has limits when parts are the fundus are illuminated by scattered light if the fundus brightness is very limited and therefore it is no longer possible to detect any fundus structures.
Measures for reducing the effect of scattered light on a cataract when using a fundus camera are known in the prior art from U.S. Pat. No. 7,147,328 A. It provides for manual adjustment options for the type and severity of a cataract. As an alternative to manual adjustment for the type and severity of a cataract, it is suggested that the cloudiness of the ocular lens be determined automatically. No specific procedure for automatically determining the severity of the cataract is provided. The imaging then occurs as a function of the severity of the cataract or cloudiness of the ocular lens. The more severe the cataract or the cloudier the eye lens, the greater the reduction made in the illumination brightness of a xenon lamp and in the output amplification in the blue channel of an imaging color image sensor should be. In this manner the scattered light saturation of the image taken is to be reduced.
It is a drawback of the suggested measure that by reducing the illumination brightness at the same exposure time, the image taken becomes darker, which has a negative impact on the contrast. If, for the sake of compensation, the exposure time is extended, the motion blur in the image increases due to unavoidable eye movements. In addition, diminishing the blue portion of light during imaging distorts the color impression of the image. Furthermore, a cataract also scatters light at wavelengths other than blue. This portion of the scattered light degrades the image quality despite the suggested measures for reducing the scattered light.
DE 101 29 652 A1 describes an arrangement and a method for determining the two-dimensional distribution of fundus pigments, especially the macular pigment xanthophyll. Here, a two-dimensional reflection image of the retina is taken in a selected narrow band wavelength range. During its evaluation, location specific areas are established for determining the optical density and comparison values. The optical density of the fundus pigment at each fundus location provides a comparative intensity value for the reflection image from the negative logarithmic value of the quotient of the intensity value of the reflection image at this fundus location. Even with this solution, it is a drawback that occurring scattered light leads to distortions of the measurements and thus of diagnosis results. No scattered light measurement and/or reduction in its effect is provided here.
Another solution for automatically determining the severity of a cataract in an eye is described in DE 10 2007 053 386 A1, in which the images of a section of an eye that are impacted by the scattered light are produced with undistorted color impression and the scattered light portion is determined. In this case, a light pattern that has at least one bright and one dark area is produced in the eye, at least a part of the dark area is imaged in a scattered light image and the intensities are integrated to create a cataract severity value. If the scattered light image is taken by use of a camera in a space-resolved manner, this represents a scattered light distribution that may be interpreted as distribution of cataract severity values. The spectrum and the brightness of an adapted illumination is adjusted for subsequent imaging as a function of the determined scattered light distribution and/or the cataract severity value.
Also suggested in DE 10 2007 047 300 A1 is a solution for determining the optical density of the macular pigment xanthophyll on the ocular fundus, using a determination based on reflection, in which solution the measurement results are not affected by interfering light, especially individual scattered light from the anterior ocular media. In this case only part of the ocular fundus is illuminated and the intensity of the interfering light from the non-illuminated area is measured in addition to the reflection light from illuminated areas. The measured value is used as a correction variable for calculating the optical density of the macular pigment.
In this case, it is a drawback that, just as in the solution described in the foregoing, one or a plurality of field stops must be present that are advantageously arranged to be able to pivot into and out of the illumination beam or measurement beam path. Such field stops entail additional costs. In addition, in this method at least two images of the fundus must be made and processed. Retrofitted integration into fundus cameras already available is not possible, or is only possible at great expense.
In addition to the lack of options to correct the scattered light for the images taken of the fundus, the solutions known from the prior art have the following other drawbacks:                Devices are needed for compensating the distortions;        In addition to the luminous field diaphragm present in every fundus camera, shading units for limiting image field illumination are need;        Information in the fundus image may be lost due to the shading units.        Additional fundus images are needed for determining the information about the scattered light.        
Not only is the increased complexity of the technical equipment associated with additional costs, but it also means that operating such equipment is more complicated, which may then have a negative impact on the reliability and/or accuracy of the measurements.